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In recent years, model systems have been extensively studied that promote the photoreduction of H 2 0 to H2 through the use 0002-7863/83/ 1505-6360$01

visible region; the Cys complex shows a discrete absorption band in the 350-380-nm region. By use of the Benesi-Hildebrand and other treatments for the variations of the absorbances of the complexes with the concentrations of the components, values of and ex for the complexes are obtained as functions of pH. Values of range from 0.29 M"1 for the Cys complex at pH 3.5 to 18 M"1 for the EDTA complex at pH 11.2; Kn values are significantly higher in alkaline solution where the deprotonated donors are better reducing agents. The emission at 525 nm observed in these and other MV2+ systems is believed to arise from a highly luminescent impurity, not a CT complex. The ramifications of CT complex formation on the photochemistry of systems containing MV2+ and sacrificial electron donors are discussed.

Electron-transfer reactions between the lowest-lying triplet state, 123 456irir*, of ruthenium (phthalocyanine)(pyridine)2 and various nitroaromatic compounds have been studied by laser and conventional flash photolysis. Quenching rate constants determined for the oxidation of the excited state have been treated according to the Marcus-Hush theory. A self-exchange rate constant k ~107 8M'1 s'1 was determined for the self-exchange reaction between the 3 *irir* and the radical cation, Ru(ph)(py)2+. Such a value indicates that the major component to the Franck-Condon reorganizational energy is the outer-sphere contribution. The photochemical properties of the phthalocyanines are discussed in terms of the redox potentials estimate for various excited states.